Quantum electronic devices which manipulate the flow of individual electrons, are known and reference is directed to W. Chen, H. Ahmed and K. Nakazato, Appl. Phys. Lett. vol. 66, 3383 (1995). These prior devices make use of so-called quantum dots which are conductive islands, constructed on the nanometre scale which exhibit Coulomb blockade to single electron tunnelling. According to this effect, the charging energy of the island is greater than the surrounding thermal energy, so that once charged by the entry of a single electron, no further electrons can enter the dot until an electron leaves. Quantum electronics devices use this effect to manipulate single electrons entering and leaving a quantum dot, and typically consist of a metal or semiconductor quantum dot in which electrons are localised, dielectric tunneling barriers through which single electrons enter or leave, and electrodes which either electrostatically bias the dot or supply electrons to the dot through the barriers.
A general discussion of the Coulomb blockade effect for such a system is given in Hermann Grabert and Michel H. Devoret, "Single Charge Tunneling" (Plenum Press, New York, 1992) pp. 1-3. From this, it will be understood that there are two basic requirements for realising the Coulomb blockade effect in a single electron device, as follows: EQU E.sub.c =e.sup.2 /2CkT, (1) EQU R.sub.T R.sub.K =h/e.sup.2 (2)
where T is the device operation temperature, R.sub.T is a tunneling resistance, R.sub.K =h/e.sup.2 is the resistance quantum, and E.sub.c =e.sup.2 /2C is the energy required to place an electron at a localised electronic state in the dot. Here, the capacitance C is defined as a constant that relates the energy of the localised state and its spatial extent i.e. the geometrical shape of the charge distribution. The capacitance C is easily defined in the case of metal clusters, and is also definable in the case of semiconductor quantum dots. Reference is directed to M. Macucci, Karl Hess and G. J. Iafrate, "Electronic Energy Spectrum and the Concept of Capacitance in Quantum Dots", Phys., Rev. B, vol. 48, p. 17354 (1993).
It is desirable that a practical single electron device be operable at room temperature i.e. with T=300.degree. K. This implies from criterion (1) listed above that the capacitance C should be very small, namely C3.times.10.sup.-18 F.
The capacitance C can be designated as the sum of two different capacitances, as follows: EQU C=C.sub.S +C.sub.e
where C.sub.S is known as the self-capacitance and is constituted by the capacitance between the island and infinity. C.sub.e is the capacitance between the dot and the surrounding electrodes of the device. When the dot diameter becomes smaller than 10 nm, the self-capacitance C.sub.S mainly determines the total capacitance C as it is much larger than the other capacitances C.sub.e. Therefore, in order to ensure room temperature operation, the dot size should be reduced below 10 nm. However, devices with a dot size of less than 10 nm cannot be fabricated readily using conventional electron beam lithography as this is smaller than the state of the art techniques can readily achieve.
It is known that nanometre scale particles can be prepared chemically, with diameters of less than 10 nm, such as colloidal particles and molecular clusters. For example, gold colloidal particles in citrate gold sols have been prepared in a size range from 2 nm to 100 nm. Reference is directed to N. Uyeda, M. Nishino, and E. Suito, J. Colloidal and Interface Science, vol. 43, p. 181 (1973). Also, ligand-stabilised Au.sub.55, clusters with a gold metal core size of 1.4 nm have been reported--see G. Schon and U. Simon, Colloid Plym. Sci., vol. 273, pp. 101-127 and pp. 202-218 (1995). One of the significant features of these materials is their well defined particle size distribution. In the case of non-dispersed colloidal Au particles as reported by Uyeda et al supra, the size distribution has a 10% standard deviation. In the case of molecular clusters, the size distribution is determined at an atomic-scale accuracy. The average diameters of these nano-scale particles are determined by the parameters of the chemical reactions that form the particles, such as the volume ratios of starting material solutions. The particle formation reactions proceed spontaneously according to the macroscopic reaction parameters and they are free of the size limitations that arise in conventional lithographic quantum dot fabrication.
A method of coating colloidal particles on metallic oxide surfaces has been known since the 1960's and reference is directed to R. K. Iller, Journal of Colloid and Interface Science, vol. 21, pp 569-594 (1966). Also, Au particle deposition on a metal oxide surface using a silane coupler was reported by A. Doron, E. Katz and I. Willmer, Langmur, vol. 11, pp 1313-1317 (1995). However, a problem with this technique was that particle coagulation occurred.
The application of cluster chemistry to single electron device fabrication is described in D. L. Klein, P. L. McEuen, J. E. B. Katari, R. Roth & A. P. Alivisatos, Pll. Phys. Lett. 68, 2574 (1996). A nanometre scale gold particle ligated alkane chains was used as a tunnel barrier. The particle was captured in a lithographically defined 5-nm gap between metal electrodes on a substrate, with limited reproducibility. This approach required complex lithographic techniques and did not use the self assembly techniques that can be achieved with cluster chemistry.
In copending EP- A-0788149 filed on Feb. 5th 1996 there is described a method of depositing nanometre scale gold particles onto a substrate in a monolayer with a controlled inter-particle spacing, suitable for use in quantum effect electronic devices. In our copending EP 96308283.9 filed on Nov. 15, 1996 and corresponding U.S. Ser. No. 08/958845 filed on Oct. 28th 1997, treating the monolayer of gold particles with an ethanolic solution of dithiol is described, to allow further overlying monolayers of the gold particles to be deposited. However, for some devices, it would be desirable to assemble nanometre scale particles in more closely packed structures such as wires, and to simplify the electrical connection of the particulate structure to other features of the device.